In many lighting applications and optical applications, for example, rear lights, brake lights of motor vehicles as well as traffic lights, monitors and RGB-applications, light sources in the deep-red range are oftentimes required.
Usually, InGaAlP-LEDs may be used, which emit light in the red spectral range. However, the LEDs come with the disadvantage that they provide good efficiency only at low temperatures, with the efficiency rapidly dropping along with an increase in temperature. Another option lies with the conversion of the radiation of blue-emitting LEDs into red light by conventional phosphors, which often have the general formula M2(Al,Si)5(N,O)8 with M=Ca, Sr, Ba and doping with europium. The problem in connection with such arrangements is that the phosphors often have good stability, but rather emit at shorter wavelengths, thus requiring very high concentrations of the phosphors to realize a deep-red lighting device with an LED as a primary light source. Such high concentrations are to ensure that re-absorption of the short-wave ranges of the converted radiation is effected by further luminescent particles, resulting in a shift of the total emission toward longer wavelengths. A high self-absorption results in high losses in terms of brightness. Further, high concentrations of the phosphor can be hardly processed.
Another option of realizing red-emitting lighting devices lies with the production of a deep-red emitting phosphor of the general formula M2(Al,Si)5(N,O)8 with M=Ca, Sr, Ba. However, this often requires high fractions of strontium as well as europium in the phosphor, which proved to be instable when exposed to radiation and temperature, for example, in LED applications.
Another option for deep-red lighting applications lies with the application of CaAlSiN3 phosphors, as described in EP 1 696 016 A1, for example. However, such phosphors exhibit good optical properties only with low dopant concentrations so that high amounts of phosphors are once again required for the realization of a deep-red application. However, in phosphor concentrations this high, scattering effects occur and, further, high phosphor concentrations can only hardly be processed.